if your deployment is integrated with AD, then the users need to exist in one of the AD domains that Tableau Server has connectivity. The alternative is local authentication. And then there are other solutions such as SAML and Open ID, but I don't know too much about these ones.

If you are server admin, click site, then Users, then Add Users to see if you are able to add an user. If yes, give the user password, try to see if you can login that user. If yes, you should be able to Import more users from CSV or using API. When use API, you can also add users to groups while Import CSV can't get users loaded into groups.